Splenic B cells are induced to proliferate upon culture with antibody having specificity for surface membrane immunoglobulins. Cells treated with pronase, washed and then cultured with antibody, exhibited a greater than 5-fold enhancement of DNA synthesis whereas pronase treatment, per se, was not mitogenic. Cells from immature mice (2 weeks of age) which essentially do not show a proliferative response to antibody become responsive subsequent to pronase treatment. In addition, splenocytes derived from strain CBA/N (x-chromosome linked immunodeficient mice) do not synthesize DNA in response to anti-mouse IgM although such cells respond readily to anti-IgM covalently linked to Sepharose. However, after pronase treatment, CBA/N splenocytes responded to free anti-IgM at 40 to 100 percent of the DNA synthetic capacity detected with Sepharose linked anti-IgM. Murine B lymphocytes in the presence of anti-IgM begin to synthesize DNA at about the 36th hour of culture although the onset of synthesis in response to other B cell reactive mitogens occurs at approximately 18 hours. By contrast, the onset of DNA synthesis by pronase treated cells in response to anti-IgM required only 18 hours. As judged from a variety of expriments pronase functions by attacking surface sites on B cells as a consequence of its proteolytic properties. Taken together the findings suggest the interpretation that a pronase sensitive site on the surface of B cells functions to regulate the proliferative response resulting from perturbation of the surface immunoglobulin receptors. Enhanced phosphorylation of protein in membranes of cells cultured with anti-IgM lilnked to Sepharose was observed initially at about 16 hours of cell culture and continued thru the 48th hour yielding the same time course as for 3H-thymidine incorporation. However, the bulk of the enlargement of cells (blast formation) occurred prior to 16 hours of culture i.e., before detection of enhanced phosphorylation or DNA synthesis. These findings suggest the possibility that membrane protein phosphorylation may play a crucial role during the committment for entry of cells into S phase.